In a conventional diesel engine, a particulate filter is incorporated in an exhaust pipe for flow of an exhaust gas to capture particulates (particulate matters) in the exhaust gas. However, the exhaust gas from the diesel engine in a normal operation status rarely has a chance to obtain a temperature level at which the particulates combust by themselves. Consequently, an oxidation catalyst having active species such as Pt and Pd is integrally carried by the particulate filter.
Specifically, employment of such particulate filter carrying the oxidation catalyst facilitates an oxidation reaction of the captured particulates to lower an ignition temperature, so that the particulates can be removed by combustion even at an exhaust gas temperature lower than ever before.
However, even if such particulate filter is employed, an amount of captured particulates will exceed an amount of treated particulates in operation areas with low exhaust temperature levels. Continued operation with such low exhaust temperature levels may hinder sufficient regeneration of the particulate filter, resulting in excessive accumulation of the captured particulates in the particulate filter.
Thus, it has been conceived to additionally arrange a flow-through type oxidation catalyst in front of the particulate filter; with accumulation of the particulates becoming increased, fuel is added to the exhaust gas upstream of the oxidation catalyst to forcibly regenerate the particulate filter.
Specifically, fuel (HC) added upstream of the particulate filter undergoes the oxidation reaction during its passage through the frontward oxidation catalyst. The exhaust gas heated by heat of the reaction and flowing into the particulate filter just behind increases a catalyst bed temperature of the particulate filter to burn off the particulates, thereby regenerating the particulate filter.
However, in a vehicle such as a city shuttle-bus with travel pattern of traveling on congested roads for a long time, the frontward oxidation catalyst hardly has an elevated catalyst bed temperature enough for sufficient catalytic activity and thus an activated oxidation reaction of the added fuel in the oxidation catalyst, failing in effective regeneration of the particulate filter within a short time.
Thus, it has been studied in recent years to arrange a burner on an entry side of the particulate filter and burn off the captured particulates by combustion with the burner regardless of an operating status of the vehicle, thereby efficiently regenerating the particulate filter within a short time.
There exist the following Patent Literatures 1 and 2 as prior art document information relating to this kind of technique on exhaust gas purification catalyst or heating of exhaust gas by use of a burner.